Perfluorinated polyethers with metal carboxylate end groups as anti-wetting and corrosion-protective agents

ABSTRACT

A process is provided for synthesizing metal salts of perfluorinated polyethers containing at least one carboxylic acid group. The polymeric salts so provided are effective as anti-wetting and corrosion-protective agents. The metal salts of perfluorinated polyether acids may be used to prepare corrosion-protected substrates, including magnetic recording disks and magnetic recording heads.

TECHNICAL FIELD

[0001] This invention relates generally to perfluorinated polyethers anduses thereof. More particularly, the invention pertains to metal saltsof perfluorinated polyethers having carboxylic acid end groups, tomethods for synthesizing the metal salts, and to use thereof asanti-wetting and/or corrosion-protective agents, e.g., in anti-wettingand/or corrosion-protective coatings on metal-containing substrates,particularly in magnetic recording devices such as magnetic recordingdisks and magnetic recording heads.

BACKGROUND

[0002] It is well known that organic carboxylic acids, R-COOH, in afluid state (liquid or vapor) primarily exist in the hydrogen bondeddimeric form, as follows:

[0003] It is also well known that the vapor phase in equilibrium withsolid NaCl at elevated temperature (T≧500° C.) is dominated by thedimeric species

[0004] The formation and stability of the latter are attributed to thepair-wise Coulombic interaction of the constituent ions; see Doan et al.(1997) J Am. Chem. Soc. 119:9810. The heat of dimerization for theacetic acid dimer has been measured to be 15 kcal/mol (Taylor (1951) JAm. Chem. Soc. 73:315; Weltner (1955) J Am. Chem. Soc. 77:3941), andthat for (NaCl)₂ to be 48 kcal/mol. The corresponding pair-wiseinteraction between salt molecules of carboxylic acids, e.g., sodiumacetate, has only recently been reported, by Doan et al. (1997), supra.

[0005] Perfluoropolyethers (PFPEs) are currently in use as lubricants ina variety of high-performance applications. PFPEs are commerciallyavailable in several distinct structural forms. Representative PFPEs areknown by the brand names Demnum® (Daikin Kogyo Co., Ltd., Japan),Krytox® (DuPont Specialty Chemicals, Deepwater, N.J.), and Fomblin® Z(Zentek SRL, Milan, Italy), having the following structural formulae

[0006] Krytox is synthesized by base-catalyzed polymerization ofhexafluoropropylene oxide, as described by Gumbrecht (1966) ASLE Trans.9:24, while Demnum is made similarly but starting with2,2,3,3-tetrafluorooxetane. The hydrogen atoms in the resulting polymersare replaced by fluorine atoms by subsequent contact with F₂ insolution, as described by Ohsaka (1985) Petrotech (Tokyo) 8:840. FomblinZ is synthesized by photooxidation of tetrafluoroethylene and is alinear, random copolymer of ethylene oxide and methylene oxide units;see Sianesi (1973) Chim. Ind. 55:208.

[0007] These PFPEs are also available with carboxylic acid end groups,as exemplified by Fomblin® Z-DIAC (Zentek SRL, Milan, Italy), Krytox®-H(DuPont) and Demnum-SH (Daikin Kogyo Co. Ltd., Japan), having thestructures

[0008] respectively. It has been found that the sodium salts of thesepolymers normally exist in the dimeric form under ambient conditions;see Doan et al. (1997), cited supra.

[0009] The inventors herein have now discovered that these and othermetal salts of perfluorinated polyethers having one or more carboxylicacid groups are extremely effective anti-wetting agents and thus findutility in a host of applications, for example in corrosion-protectivefilms. Although Doan et al. describes a method for synthesizing sodiumsalts of PFPE acids, the method described is problematic. That is, Doanet al. describes preparation of sodium salts of PFPE acids by reacting aPFPE acid (e.g., Fomblin® Z-DIAC, Krytox-He or Demnum®-SH) with a sodiumhydroxide solution, and then extracting the salt from the resultingemulsion with a fluorocarbon solvent. Although the intended product maybe prepared using this technique, the method requires handling amultilayer fluid including a viscous interfacial gel layer, a cumbersomeprocess that requires extreme care. This invention is in part directedto a new method for synthesizing metal salts of perfluorinated polyetheracids that overcomes the aforementioned disadvantage of the Doan et al.synthesis.

[0010] The invention is also premised on the discovery that metal saltsof PFPE acids are useful as anti-wetting agents and corrosion-protectiveagents. In this regard, it should be pointed out that certainfluorinated polymers, particularly poly(fluoroalkylacrylates) andpoly(fluoroalkylmethacrylates), have been used as oil and waterrepellent agents (see, for example, B.E. Smart, “Organic FluorineCompounds” in Kirk-Othmer Encyclopedia of Chemical Technology, ThirdEdition, Vol. 10, John Wiley & Sons, New York, 1980, p. 869). However,the adhesion force obtained using these polymers as coating agents isinsufficient to provide sufficient durability in many contexts.

[0011] One important application of the present compounds that exploitsthe newly discovered properties is as corrosion-protective agents thatbond strongly to metal and metal oxide substrates, as the compoundsadhere well to metal-containing substrates. Furthermore, the compoundsof the invention are extremely useful as corrosion-protective agents formagnetic recording disks and magnetic recording heads, particularlythose having a carbon overcoat. Such overcoats are typically formed bysputter deposition from a graphite target, and are generally calledprotective carbon overcoats, “diamondlike” carbon overcoats, amorphouscarbon overcoats, or, in the case of those overcoats formed by sputterdeposition in the presence of a hydrogen-containing gas, hydrogenatedcarbon overcoats. Tsai et at. in “Structure and Properties of SputteredCarbon Overcoats on Rigid Magnetic Media Disks,” J Vac. ScienceTechnology A6(4), July/August 1988, pp. 2307-2314, describe suchprotective carbon overcoats and refer to them as amorphous “diamondlike”carbon films, the “diamondlike” referring to their hardness rather thantheir crystalline structure. IBM's U.S. Pat. No. 4,778,582 describes aprotective hydrogenated disk carbon overcoat formed by sputtering agraphite target in the presence of Ar and hydrogen (H₂). The carbonovercoats may also be formed by plasma-enhanced chemical vapordeposition (CVD) and may include nitrogen in addition to hydrogen, asdescribed by Kaufman et al. (1989) Phys. Rev. B 39:13053.

[0012] To increase the areal density of the data magnetically recordedon the disk, the recording head must be brought close to the magneticlayer, which means that the overcoat thickness must be substantiallyreduced, i.e., to less than 5 nm in future disk drives. Consequently, animportant challenge faced by the disk drive industry is how to makeprotective disk overcoats that are ultra-thin yet still provide thedesired durability and corrosion protection. However, the carbonovercoat sputter-deposited on the magnetic layer of storage disks oftenabounds with pinholes, through which the corrosion of metals in themagnetic and other underlayers may occur. Reducing the thickness of thecarbon overcoat exacerbates the problem. The same drawback isencountered with the metallic elements of magnetic recording heads thatare coated with a layer of sputtered carbon. Because metal salts of PFPEacids adhere well to metal-containing surfaces and poorly to carbonovercoats, the compounds are able to fill pinholes in the protectiveovercoat without adding any substantial thickness to the disk.Anti-corrosion strength is further enhanced by the exceptional waterrepellency of PFPE acid salts.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is a primary object of the invention to addressthe above-mentioned need in the art by providing a method forsynthesizing metal salts of PFPE acids.

[0014] It is another object of the invention to provide such a methodwhich involves treating a perfluorinated polyether having at least onecarboxylic acid group with a metal salt of a volatile organic acid, andthen volatilizing the resulting organic acid.

[0015] It is another object of the invention to provide a method fortreating a metal-containing substrate to enhance water repellency andcorrosion resistance, wherein the method involves depositing acomposition containing a metal salt of a perfluorinated polyether acidonto the substrate.

[0016] It is an additional object of the invention to provide such amethod wherein the metal-containing substrate has a metal surface or asurface comprised of a metal oxide.

[0017] It is a further object of the invention to provide acorrosion-protected magnetic recording disk comprised of a substrate, amagnetic layer, and an amorphous carbon overcoat on the magnetic layerthat has been treated with a composition containing a metal salt of aperfluorinated polyether acid as a corrosion-protective agent.

[0018] It is yet a further object of the invention to provide acorrosion-protected magnetic recording head having an amorphous carbonovercoat that has been treated with a composition containing a metalsalt of a perfluorinated polyether acid.

[0019] Additional objects, advantages and novel features of theinvention will be set forth in part in the description which follows,and in part will become apparent to those skilled in the art uponexamination of the following, or may be learned by practice of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 schematically illustrates a cross-section of a magneticrecording disk treated with a corrosion-protective compositioncontaining a metal salt of a PFPE acid as a corrosion-protective agent.

[0021]FIG. 2 schematically illustrates a cross section of a treatedsubstrate wherein PFPE acid salts as provided herein fill the pinholespresent in a sputtered carbon layer.

[0022]FIG. 3 shows the IR spectra of Fomblin Z-DIAC and its sodium salt,as described in Example 1.

[0023]FIG. 4 shows the IR spectra of Demnum-SH and its sodium salt, alsoas described in Example 1.

[0024]FIG. 5 shows the IR spectra of the sodium salt of Demnum-SH before(upper spectrum) and after (lower spectrum) rinsing withtrifluoroethanol (TFE), as described in Example 2.

[0025]FIG. 6 shows the IR spectra of the sodium salt of Fomblin Z-DIACbefore (upper spectrum) and after (lower spectrum) rinsing withtrifluoroethanol (TFE), as described in Example 2.

[0026]FIG. 7 shows the IR spectra of the sodium salt of Demnum-SHadsorbed onto an aluminum plate (upper spectrum) and onto a substratesurface composed of a nickel-iron (NiFe) alloy, as described in Example3.

[0027]FIG. 8 shows the IR spectra of the sodium salt of Fomblin Z-DIACpresent as a film on a bare aluminum plate (upper spectrum) and on analuminum plate coated with a carbon film (lower spectrum), as describedin Example 4.

[0028]FIG. 9 shows the microscopic images of corrosion debris developedby cerium etching of the magnetic disks prepared in Example 4. The upperimage illustrates the results obtained with the bare carbon-coatedmagnetic disks, while the lower image illustrates the results obtainedwith the disks treated with a corrosion-protective compositioncontaining a metal salt of a perfluoropolyether acid as described inExample 5.

[0029]FIG. 10 shows the microscopic images of corrosion debris developedby exposing to an atmosphere equilibrated with 1 N HCl solution (a) anuntreated magnetic recording head assembly having an amorphous carbonovercoat (left side), (b) a magnetic recording head assembly having anamorphous carbon overcoat treated with a corrosion-protectivecomposition containing the sodium salt of Demnum-SH (upper right), and(c) a magnetic recording head assembly having an amorphous carbonovercoat treated with a corrosion-protective composition containing thesodium salt of Fomblin Z-DIAC (lower right), as described in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

[0030] Before describing the present invention in detail, it is to beunderstood that this invention is not limited to specific process steps,substrates, magnetic recording devices, or the like, as such may vary.It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting.

[0031] It must be noted that, as used in this specification and theappended claims, the singular forms “a,” “and,” and “the” include pluralreferents unless the context clearly dictates otherwise. For example, “asalt” refers to a mixture of salts as well as a single salt, “a polymer”(e.g., “a PFPE”) refers to a mixture of polymers (e.g., PFPEs) as wellas a single polymer (e.g., a PFPE), and the like.

[0032] The compounds to which the present invention is addressed aremetal salts of perfluorinated polyethers having at least one carboxylicacid end group. These salts are also referred to herein as “metal saltsof a PFPE acid,” as “PFPE acid salts” and as compounds having theformula “PFPE-COO⁻M⁺” (wherein M is the metal atom of the salt), itbeing understood that the perfluorinated polyether PFPE can contain morethan one —COO⁻M⁺moiety and possibly some degree of hydrogenation aswell. A novel synthesis for preparing such salts is provided, which, asnoted above, overcomes the disadvantages inherent in the prior art. Thesalts are synthesized by treating a perfluorinated polyether having atleast one carboxylic acid end group with a metal salt of a volatileorganic acid under reaction conditions effective to convert thecarboxylic acid group(s) of the perfluorinated polyether acid to thesalt form and volatilize the resulting organic acid, resulting in areaction product comprising a metal salt of the PFPE acid. Theperfluorinated polyether that serves as the starting material iscomprised of monomer units having the structure —CF₂—O—, —CF₂—CF₂—O—,—CF₂—CF₂—CF₂—O—, —CF(CF₃)—O—, —CF(CF₃)—CF₂ O—, or a combination thereof,and has at least one carboxylic acid group, generally at one terminus ofa substantially linear polymer, and typically has two carboxylic acidgroups, one present at each terminus of a substantially linear polymer.Such polymers include, but are not limited to, the commerciallyavailable polymers Fomblin® Z-DIAC, Krytox®-H and Demnum®-SH. Althoughreferred to herein as a “perfluorinated” polyether, the polymer may bepartially hydrogenated, in which case up to about 50% of the fluorineatoms in the perfluorinated polyether are substituted with a hydrogenatom. The polymeric acid that serves as the starting material in thesynthetic process generally has a number average molecular weight in therange of approximately 500 to 10,000, preferably 1000 to 5000, mostpreferably 2500 to 3500.

[0033] The metal salt of the volatile organic acid has the structureRCOO-M+where M is the metal, preferably an alkali metal, e.g., sodium,and R is hydrocarbyl, typically alkyl, and preferably lower alkyl. Thus,an exemplary alkali metal salt is sodium acetate, CH₃COONa. The presentmethod exploits the fact that the organic acid component of the alkalimetal salt is volatile; for example, acetic acid has a boiling point of118° C. As noted above, the perfluorinated polyether starting materialis reacted with the metal salt of the volatile organic acid underreaction conditions effective to convert carboxylic acid groups to thesalt form and volatilize the resulting organic acid. Suitable reactionconditions involve heating a mixture of the perfluorinated polyetheracid and the metal salt of the volatile organic acid at a temperature ofat least about 130° C. for at least 48 hours. Preferably, a fluorinatedsolvent such as perfluorohexane (e.g., FC72® 3M, St. Paul, Minn.) isthen added to the reaction mixture and the mixture is heated at refluxuntil smooth. Additional fluorinated solvent may be added at this point;in addition, a lower alkanol such as methanol may be added to removeunreacted metal salt, and trifluoroethanol may be added to minimize gelformation. The reaction product is then isolated by extraction, e.g.,using a lower alkanol such as methanol. Specific examples describingsynthesis of the sodium salt of Demnum-SH and Z-DIAC, respectively, areincluded herein as Examples 1 and 2.

[0034] These PFPE acid salts may be applied to a substrate surface toform a film thereon. Alternatively, a pinhole-containing layer on asubstrate—e.g., an amorphous carbon coating on a metallic substrate—maybe treated with a solution of a PFPE acid salt such that the pinholesbecome filled with the salt solution, preventing corrosion of theunderlying metal-containing layer or layers. The compounds prepared assynthesized above have been found to be extremely effectivewater-repellent and oil-repellent agents, and adhere well tometal-containing surfaces, e.g., metallic surfaces or surfaces of metaloxides. Adhesion to an aluminum surface is described in Example 3, andadhesion to a NiFe alloy surface is described in Example 4.

[0035] One important area in which the present compounds find utility isas corrosion-protective layers in magnetic storage devices such asmagnetic storage disks and magnetic recording heads. Of particularinterest are magnetic storage disks and magnetic recording heads thathave an overcoat of essentially amorphous carbon, as disclosed, forexample, in U.S. Pat. No. 5,030,494 to Ahlert et al. and U.S. Pat. No.5,075,287 to Doemer et al., both assigned to IBM Corporation. Asexplained in the aforementioned patents, many rotating rigid disk drivesinclude read/write transducers (or “heads”) supported on a carrier (or“slider”) that ride on a cushion or gearing of air above the surface ofa magnetic recording disk when the disk is rotating at operating speed.The slider has an air-bearing surface (“ABS”), typically in the form ofa plurality of rails, and is connected to a linear or rotary actuator bymeans of a suspension. There may be a stack of disks in the disk drivewith the actuator supporting a number of sliders. The actuator moves thesliders radially so that each head may access the recording area of itsassociated disk surface. The slider in the disk drive is biased towardthe disk surface by a small force from the suspension. The ABS of theslider is thus in contact with the disk surface from the time the diskdrive is turned on until the disk reaches a speed sufficient to causethe slider to ride on the air bearing. The ABS of the slider is again incontact with the disk surface when the disk drive is turned off and therotational speed of the disk fails below that necessary to create theair bearing. This type of disk drive is called a contact start/stop(CSS) disk drive. To provide wear resistance for the ABS in a CSS diskdrive, a protective carbon overcoat may be placed on the slider rails.IBM's U.S. Pat. No. 5,159,508 describes a slider with air-bearing railshaving an amorphous carbon overcoat that is adhered to the rails by asilicon adhesion layer.

[0036] The magnetic recording disk in a CSS rigid disk drive istypically a thin film disk comprising a substrate, such as a disk blankmade of glass, ceramic, glassy carbon or an aluminum-magnesium (AlMg)alloy with a nickel-phosphorous (NiP) surface coating, and acobalt-based magnetic alloy film formed by sputter deposition over thesubstrate. A protective overcoat, such as a sputter-deposited amorphouscarbon film, is formed over the magnetic layer to provide corrosionresistance and wear resistance from the ABS of the slider. The overcoatmay further include relatively small amounts of embedded iron (Fe),tungsten (W) or tungsten carbide (WC) to improve wear resistance andminimize the likelihood of damage to disk file components (see U.S. Pat.No. 5,030,494 to Ahlert et al., cited above). Such overcoats aretypically formed by sputter deposition from a graphite target, and asexplained in the Background section, are generally called protectivecarbon overcoats, “diamondlike” carbon overcoats, amorphous carbonovercoats, or, in the case of those overcoats formed by sputterdeposition in the presence of a hydrogen-containing gas, hydrogenatedcarbon overcoats. In addition to the magnetic layer and the protectiveovercoat, the thin film disk may also include a sputter-depositedunderlayer, such as a layer of chromium (Cr) or a chromium-vanadium(CrV) alloy, between the substrate and the magnetic layer, and asputter-deposited adhesion layer, such as a Cr, tungsten (W) or titanium(Ti) layer, between the magnetic layer and the protective overcoat.

[0037] As alluded to above, a problem associated with the aforementionedprotective overcoat is that the sputter-deposited carbon layer oftenabounds with pinholes, through which the corrosion of metals in themagnetic and other underlayers may occur. These pinholes can be detectedin a number of ways, for example using a cerium etching technique, whichemploys (NH₄)₂Ce(NO₃)₆ as an oxidizing agent and selectively oxidizesmetal, e.g. chromium metal, in the underlayer. Corrosion debris evolvedat pinhole sites may thus be observed and counted with an opticalmicroscope. The present compounds, since they adhere well to metal andmetal oxide surfaces but do not adhere to amorphous carbon, can beapplied to the surface of the amorphous carbon overcoat so as to fill inthe pinholes and eliminate or at the very least minimize the possibilitythat the underlying layers may corrode.

[0038] Thus, in one embodiment, an improved magnetic recording disk isprovided that is fabricated by treatment with a metal salt of aperfluorinated polyether acid. At a minimum, the magnetic recording diskcomprises: a substrate; a magnetic layer formed over the substrate; anovercoat formed over the magnetic layer, the overcoat being a filmhaving a substantially planar surface and comprising primarily carbon inthe essentially amorphous form and optionally one or more of tungsten ortungsten carbide distributed throughout and embedded within the carbon;and a corrosion-protective composition filling pinholes in the carbonovercoat, the corrosion-protective composition containing a metal saltof a PFPE acid as a corrosion-protective agent. Optimally, the magneticlayer is comprised of a cobalt-based magnetic alloy film formed bysputter deposition over the substrate; a particularly preferredcobalt-based magnetic alloy is CoPtCrB, as described in IBM's U.S. Pat.No. 5,523,173 to Doerner et al.

[0039] A thin film disk 10 according to the present invention isillustrated in section in FIG. 1. The disk 10 includes a substrate 12,typically comprising a disk blank made of glass, ceramic, glassy carbonor an aluminum-magnesium (Al-Mg) alloy with a nickel-phosphorous (Ni-P)surface coating. A chromium (Cr) or a chromium-vanadium (Cr-V) alloy 14underlayer is sputter-deposited on the substrate. Over the underlayer 14is deposited a magnetic layer 16, which preferably, as explained above,is comprised of a cobalt-based magnetic alloy such as CoPtCrB. Over themagnetic layer 16 is overcoat 18 of sputter-deposited amorphous carbon,containing pinholes 20. The pinholes 20 are filled with acorrosion-protective composition 22 containing a PFPE acid salt; fillingthe pinholes in this way prevents exposure of the underlying magneticlayer. Magnetic head 24 is mounted on arm 26, which is connected tomeans (not shown) for positioning head 24 in a generally radialdirection with respect to disk 10. FIG. 2 illustrates more specificallyhow molecules of the PFPE acid salt 28, with polar, ionic end group 30,fill the pinholes 20 in the amorphous carbon overcoat 18, protectingareas 32 in the underlying metal-containing magnetic layer 16 that wouldotherwise be susceptible to corrosion. It may be desirable to then coatthe pinhole-filled surface with a layer of a perfluoropolyetherlubricant (not shown).

[0040] As noted above, the PFPE metal salts of the invention are alsouseful in providing corrosion protection for magnetic recording headsand associated head assembly components, particularly when asputter-deposited amorphous carbon overcoat is used as described withrespect to magnetic recording disks. A magnetic recording head assemblyof the invention will generally include at least one component comprisedof an oxide, nitride or carbide of aluminum, zirconium, silicon ortitanium and have an overcoat of sputter-deposited amorphous carbon,wherein the improvement lies in the use of a corrosion-protective agentof the invention to fill pinholes present in the carbon overcoat andprevent exposure of the underlying layer.

[0041] It is to be understood that while the invention has beendescribed in conjunction with the preferred specific embodimentsthereof, that the foregoing description as well as the examples whichfollow are intended to illustrate and not limit the scope of theinvention. Other aspects, advantages and modifications within the scopeof the invention will be apparent to those skilled in the art to whichthe invention pertains.

[0042] All patents, patent applications, and publications mentionedherein are hereby incorporated by reference in their entireties.

[0043] Experimental:

[0044] The following examples are put forth so as to provide those ofordinary skill in the art with a complete disclosure and description ofhow to prepare and use the oligomers and polymers disclosed and claimedherein. Efforts have been made to ensure accuracy with respect tonumbers (e.g., quantities, temperature, etc.) but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C. and pressure is at or nearatmospheric. Additionally, all starting materials were obtainedcommercially or synthesized using known procedures.

EXAMPLE 1

[0045] Synthesis of Metal Salts of PFPE Acids:

[0046] (a) Demnum-SH: The sodium salt of Demnum-SH was prepared asfollows. Twenty-five (25) grams of Demnum-SH (Daikin Kogyo Co., Ltd.,Japan) and 2 grams of sodium acetate were mixed in a 100 ml beaker. Themixture was stirred at 140˜150° C. with stirring. The mixture bubbledand became rigid within about an hour. Heating at 145° C. was continuedfor three days until bubbling stopped completely. The reaction mixturewas then transferred into an 250 cc Erlenmeyer flask, and 50 cc of FC72(perfluorohexane) were added. A condenser was attached, and the mixturewas refluxed at 60° C. until smooth. The mixture was then transferredinto a 250 cc separatory funnel, and 50 cc of FC72 (additional), 50 ccof methanol (for removal of excess Na-acetate), and 10 cc of TFE(trifluoroethanol, CF₃—CH₂—OH) were added to minimize gel formation. Themixture was shaken in the separatory funnel vigorously and then allowedto stand overnight. Two clear layers developed. The upper methanol layercontained excess sodium acetate. The lower layer (˜100 cc) contained thedesired Demnum-SH salt. The lower layer was collected in a deepcontainer, and heated gently (˜50° C.) overnight to remove the solvent.The product was then dried in a vacuum oven maintained at ˜100° C.overnight.

[0047] (b) The sodium salt of Fomblin Z-DIAC (obtained from AusimontUSA, Inc.) was prepared following essentially the same procedure. Inthis case, however, after the separatory funnel process, only ˜50% (byweight) of the expected product was recovered from the FC72 layer. Thebalance of the material was found in the CH₃₀H layer. An IR analysisproved that both fractions were the Na salt of Fomblin Z-DIAC. An F-19NMR analysis revealed, however, that the number average molecular weightof the CH₃OH fraction was 1500, while that of the FC72 fraction was3000. The number averaged molecular weight of the starting FomblinZ-DIAC was 2000. The molecular weight of the starting Demnum-SH was3000. Since Demnum-SH is monofunctional, while Z-DIAC is bifunctional,it was concluded that in the synthesis of the Na salt of Demnum-SH, verylittle product molecules resulted that were small enough to be solublein methanol. It was further concluded that, for the Na salts of FomblinZ-DIAC of molecular weight ˜1500, the polarity rendered by theNa-carboxylate groups attached at both ends of a molecule is such thatit makes the entire molecule soluble in methanol.

[0048]FIG. 3 shows the IR spectra of the starting Fomblin Z-DIAC and itsNa salt of molecular weight 3000. FIG. 4 shows the IR spectra ofstarting Demnum-SH and its Na salt. In both cases the product integrityis attested by the complete shift of the carbonyl band from ˜1780 to˜1680 cm⁻¹; see Doan et al. (1997) J Am. Chem. Soc. 119:9810.

[0049] For simplicity, the abbreviations Z(COONa)₂ and D-COONa will beused hereinafter to indicate respectively the Na salts of Fomblin Z-DIACand Demnum-SH thus synthesized. Unless mentioned otherwise, Z(COONa)₂refers to the product obtained in the FC72 fraction (with the numberaveraged molecular weight of ˜3000), as does D-COONa (also with thenumber averaged molecular weight of 3000).

[0050] It was found that both Z(COONa)₂ and D-COONa were sparinglysoluble in FC72 (perfluorohexane; b.p. 58° C.) but were quite soluble inTFE (trifluoroethanol, b.p. 77° C.) and also in HFE-A(nonafluoro(iso)butyl-methyl ether; b.p. 60° C.).

EXAMPLE 2

[0051] Adhesion of Z(COONa)₂ and D-COONa on an Aluminum Surface:

[0052]FIG. 5 shows the IR spectra (obtained by the reflectancetechnique) of Z(COONa)₂ applied as a thin smear on an aluminum plate(upper spectrum) and after the same plate was rinsed thoroughly withpure TFE (lower spectrum). The corresponding spectra observed withD-COONa are shown in FIG. 6. The spectra observed after the TFE rinsewere attributed to a molecular monolayer of the salt molecules adheringto the aluminum surface. Essentially the same spectrum was observed froman aluminum coupon which was dipped into a TFE solution of Z(COONa)₂(0.3 wt. %) and then rinsed thoroughly with pure TFE.

[0053] A further study revealed that these monolayers of Z(COONa)₂ orD-COONa adhering to the aluminum surface were not removed when thesample coupons were immersed in water with stirring for one hour, norwhen they were immersed in TFE with stirring for one hour.

[0054] The thickness of the D-COONa film remaining on an aluminum plateafter the dip-and-rinse process was measured by the ellipsometrytechnique and determined to be 70 A. The contact angle (of waterdroplet) of the aluminum plate increased from 43° to 115° upon adhesionof D-COONa by the dip-and-rinse process. The contact angle determinedfor the treated plate was close to that of a Teflon-coated surface. Itis thus strongly suggested that adhesion occurs solely due to theinteraction between the polar sodium-carboxylate unit and the polarmetal surface, with the nonpolar perfluoropolyether moiety extendingfreely outward.

[0055] As expected, the aluminum surface thus treated was extremelywater repellent and oil repellent. The treated surface does not permitwriting with either a water-based pen or an oil-based pen.

EXAMPLE 3

[0056] Adsorption on Other Metal Surfaces:

[0057] In order to demonstrate that the observed strong adsorption ofthe Na salts of PFPE-acids was not uniquely limited to the aluminumsurface as described in Example 2, adsorption of D-COONa upon a NiFealloy surface was examined. FIG. 7 compares the spectra observed from analuminum plate and a NiFe disk, both of which had been dipped into a TFEsolution of D-COONa (0.3 wt. %) and then thoroughly rinsed with pureTFE. It was revealed that a substantially identical quantity of D-COONamolecules adhered to the NiFe surface.

EXAMPLE 4

[0058] Interaction between Na-PFPE Carboxylate and Sputtered Carbon:

[0059] In order to verify that the observed strong adsorption of D-COONaor Z(COONa)₂ to metal surfaces was due to the Coulombic interactionbetween the extremely polar (ionic) Na-carboxylate end group of thepolymer molecule and the polar constituents of the metal (oxide)surface, the adsorption of Z(COONa)₂ upon aluminum plates coated with acarbon film of thickness ˜200 A was evaluated. The IR spectra observedfrom bare and carbon coated aluminum plates which had been soaked in aTFE solution of Z(COONa)₂ (1%) for three hours followed by thoroughrinsing with pure TFE are compared in FIG. 8. The immersion process wasused in order to ensure (possible) bonding to a porous carbon medium. Itwas revealed that only a trace amount of D-COONa was adsorbed on thecarbon coated aluminum. The ellipsometry measurement showed the presenceof Z(COONa)₂ corresponding to a thickness of 5 Å. It was surmised thatthe small amount of adsorption occurred through pinholes of the carbonfilm.

EXAMPLE 5

[0060] Sealing Pinholes of Carbon Overcoat on Magnetic Storage Disk:

[0061] Since Na-PFPE carboxylate molecules adhere strongly to a metaloxide surface but not to the surface of sputtered carbon, Na-PFPEcarboxylate molecules adhere to the metal oxide layer of a magneticstorage disk through pinholes in a sputter-deposited amorphous carbonovercoat. FIG. 9 compares the microscopic images of corrosion debrisdeveloped by the cerium etching technique on two disks of the same lot.The cerium etching technique uses (NH₄)₂Ce(NO₃)₆ as an oxidizing agentand selectively oxidizes the underlayer. Corrosion debris evolved atpinhole sites may thus be observed and counted with an opticalmicroscope. Prior to etching, one of the disks was dipped into a TFEsolution of D-COONa (1%) followed by a thorough rinsing with pure TFE.It was revealed that the pinhole density was reduced by a factor ofthree by the dip-and-rinse process of a D-COONa/TFE solution.

EXAMPLE 6

[0062] Corrosion Protection of GMR Heads:

[0063] It is particularly essential that the metallic elements of GMR(giant magnetoresistance) heads in magnetic storage disk systems areprotected from corrosion. To that end, an entire head assembly wascoated with a layer of sputtered carbon. The corrosion, however, couldstill develop through pinholes of the carbon overcoat. It was found thatby exposing an HGA (the head and gimbals assembly) to an atmosphereequilibrated with 1.0 N HCl solution for thirty minutes, a sufficientamount of corrosion debris developed that was observable by an opticalmicroscope. FIG. 10 shows, on the left side, the images of several headsthat have been subjected to this accelerated HCl vapor test. Shown onthe right side are the images of heads that had been first immersed in aTFE solution of D-COONa (1%) or Z(COONa)₂ (1%), rinsed sequentially withpure TFE, FC72, and IPA (isopropyl alcohol), and then subjected to theHCl vapor test. The efficacy of the immersion in a Na-PFPE carboxylate,solution is evident. We surmise, as before, that pin holes in the carbonlayer are effectively sealed by Na-PFPE carboxylate molecules, andhydrophobic PFPE chains extending outward block intrusion of polarmolecules such as H₂0 and HCl.

1. A process for preparing a metal salt of a perfluorinated polyetherhaving at least one carboxylic acid end group, comprising: treating aperfluorinated polyether having at least one carboxylic acid end groupwith a metal salt of a volatile organic acid under reaction conditionseffective to convert all carboxylic acid end groups to the salt form andvolatilize the resulting organic acid, thus providing a reaction productcomprising a salt of the metal and the perfluorinated polyether.
 2. Theprocess of claim 1, wherein the perfluorinated polyether is comprised ofmonomer units having the structure —CF₂—O—, —CF₂—CF₂—O—, —CF(CF₃)—O—,—CF(CF₃)—CF₂—O—, or a combination thereof.
 3. The process of claim 1,wherein the perfluorinated polyether is partially hydrogenated.
 4. Theprocess of claim 3, wherein up to about 50% of the fluorine atoms in theperfluorinated polyether are substituted with a hydrogen atom.
 5. Theprocess of claim 1, wherein the perfluorinated polyether is a linearpolymer.
 6. The process of claim 5, wherein the perfluorinated polyetherhas a single carboxylic acid end group.
 7. The process of claim 5,wherein the perfluorinated polyether has two carboxylic acid end groups.8. The process of claim 1, wherein the metal salt is an alkali metalsalt.
 9. The process of claim 8, wherein the alkali metal salt is asodium salt.
 10. The process of claim 1, wherein the volatile organicacid is acetic acid.
 11. The process of claim 1, wherein the reactionconditions comprise heating a mixture of the perfluorinated polyetherand the metal salt of a volatile organic acid at a temperature of atleast about 130° C. for at least 48 hours.
 12. The process of claim 1,further comprising isolating the reaction product.
 13. The process ofclaim 12, wherein the product is isolated by extraction.
 14. The processof claim 13, wherein the extraction employs a fluorinated alkane solventand a lower alkanol.
 15. The process of claim 14, wherein the extractionemploys perfluorohexane and methanol.
 16. The process of claim 1,wherein the perfluorinated polyether has a number average molecularweight in the range of approximately 500 to 10,000.
 17. The process ofclaim 16, wherein the perfluorinated polyether has a number averagemolecular weight in the range of approximately 1000 to 5,000.
 18. Theprocess of claim 17, wherein the perfluorinated polyether has a numberaverage molecular weight in the range of approximately 2500 to
 3500. 19.A metal salt of a perfluorinated polyether having at least onecarboxylic acid end group, prepared by the process of claim
 1. 20. Amethod for making a magnetic recording disk comprising: providing a disksubstrate; forming an underlayer on the substrate; sputter-depositing acobalt-based alloy magnetic layer on the substrate; sputter-depositing acarbon overcoat onto the magnetic layer; and applying to the carbonovercoat a corrosion-protective composition containing a metal salt of aperfluorinated polyether acid, thereby filling any pinholes in thecarbon overcoat with the corrosion-protective composition.
 21. Anarticle of manufacture comprised of a substrate having a metal oxidesurface coated with a layer of a metal salt of a perfluorinatedpolyether having at least one carboxylic acid end group.
 22. An articleof manufacture comprised of a substrate having a metal surface coatedwith a layer of a metal salt of a perfluorinated polyether having atleast one carboxylic acid end group.
 23. A corrosion-protected magneticrecording disk, comprising: a substrate; a metal or metal oxide magneticlayer formed over the substrate; an overcoat formed over the magneticlayer, the overcoat comprising primarily carbon in the essentiallyamorphous form and having pinholes therein exposing the magnetic layer;and a corrosion-protective agent comprised of a metal salt of aperfluorinated polyether having at least one carboxylic acid group,filling the pinholes of the overcoat.
 24. The magnetic recording disk ofclaim 23, wherein the magnetic layer is comprised of a cobalt-basedalloy.
 25. The magnetic recording disk of claim 24, wherein thecorrosion-protective agent comprises a metal salt of a perfluorinatedpolyether having two carboxylic acid groups.
 26. The magnetic recordingdisk of claim 23, further comprising a chromium-containing underlayerbetween the magnetic layer and the substrate.
 27. The magnetic recordingdisk of claim 23, further comprising a lubricating film of aperfluoropolyether on the overcoat.
 28. In a magnetic recording headcomprised of (a) a substrate of an oxide, nitride or carbide ofaluminum, zirconium, silicon or titanium and having (b) an overcoatcomprising a film composed primarily of carbon in the essentiallyamorphous form and having pinholes therein exposing the substrate,wherein the improvement comprises treating the magnetic recording headwith a corrosion-protective composition so as to fill the pinholes ofthe overcoat, the composition comprised of a metal salt of aperfluorinated polyether having at least one carboxylic acid group. 29.The magnetic recording head of claim 28, wherein thecorrosion-protective agent comprises a metal salt of a perfluorinatedpolyether having two carboxylic acid groups.